Mapping the magnetic field of our galaxy’s supermassive black hole

A pulsar at the Milky Way's core lets us track the field's strength.

Artist's impression of a pulsar (the smaller object at the right) orbiting the central black hole of the Milky Way. The beams of light represent the pulses, which are affected by the magnetic field surrounding the black hole.

Bill Saxton, NRAO/AUI/NSF

The black hole at the center of the Milky Way is a bright source of radio and X-ray light. According to theory, this light is emitted due to hot gas orbiting and falling into the black hole. However, the shape of the gas flow is dictated by the magnetic fields at the galactic center, which are difficult to measure.

Astronomers hoped fondly for the discovery of a pulsar, a rapidly rotating remnant of a dead star that generates powerful jets of radio photons. Any magnetic field near the pulsar would affect the radio emissions, allowing researchers to map the environment near the black hole.

The wish was granted earlier this year when an X-ray flare revealed the presence of a pulsar within one light-year of the Milky Way's black hole. R.P. Eatough and colleagues measured the light from the pulsar and found it was strongly rotated, a sure sign of a magnetic field. If the magnetic field they detected extends to the surface of the black hole, it would be sufficient to explain the entire spectrum of emissions from the Milky Way's center.

Astronomers have observed extraordinarily massive black holes at the center of most galaxies. Those black holes are evident through X-rays and radio waves emitted by the plasma swirling around them, but the mechanisms that drive the emissions aren't always obvious. According to theory, magnetic fields shape the motion of the matter, accelerating it in some instances and removing its energy in others, ultimately allowing it to be swallowed by the black hole.

The problem with testing this hypothesis: we can't directly see magnetic fields. While they affect the motion of electrically charged particles such as electrons, the trajectories of these particles are impossible to see at such great distances. (The Milky Way's black hole is about 27,000 light-years away from Earth.) The best hope is measuring Faraday rotation, the twisting of the polarization of light when it passes through a strong magnetic field. Light from matter near the Milky Way's center is strongly rotated, but that only provides data close to the black hole—insufficient to study the total magnetic field at the galaxy's center.

Pulsars are neutron stars, the remnants of the cores of stars more than 8 times the mass of the Sun. They send out beams of intense radio light, which appear as regular pulses of light to observers on Earth thanks to the star's rapid rotation. The timing of the pulses reveals the neutron star's rotation rate. Additionally, the light in the pulses are polarized, so if they pass through a magnetic field, they'll be Faraday rotated. By measuring the amount of rotation, researchers can map the strength and direction of the magnetic field.

(Light's polarization is the orientation of the electric and magnetic field of a photon. Most sources, including stars, produce unpolarized light: each photon's polarization is random, so the light collectively has no preferred orientation. Pulsars, on the other hand, generate strongly polarized radio waves.)

The researchers found the pulsar—named PSR J1745-2900 in the usual annoying fashion—was rotating once every 3.76 seconds. By measuring the difference in arrival times between the different frequencies of radio emissions, they determined the density of electrons (which scatter the photons depending on their energy) surrounding the pulsar. This density indicated how close the pulsar is to the galactic center: its maelstrom of hot magnetized plasma must be closer than about 33 light-years from the central black hole.

Additionally, the astronomers measured the Faraday rotation of the pulsar's light using three different radio observatories. They consistently found a rotation measure more than 10 times larger than seen elsewhere in the galaxy. That's only possible in the presence of greater-than-average magnetic fields, such as predicted near a massive black hole. These magnetic field measurements would be sufficient to explain all the emissions near the central black hole, so long as the field behaves as predicted by theory.

As PSR J1745-2900 orbits the galactic center, astronomers should be able to measure changes in its environment, providing more details about the magnetic field and plasma near the galaxy's core.

PSR J1745-2900 was first identified when it produced a strong X-ray flare. Combined with its rotation rate, that marked it as a magnetar: a pulsar with a magnetic field 1,000 times stronger than ordinary neutron stars. Magnetars are rare, so the chances of PSR J1745-2900 being the only neutron star in the region are very small. That provides hope of finding more pulsars, perhaps even closer to the black hole. With a population to study, it should be possible to map the Milky Way's magnetic field in detail, including the region near the black hole where theory predicts turbulent behavior.

Additionally, a pulsar closer in to the black hole would experience relativistic effects as its photons pass through the strong gravitational field. Such measurements would provide a detailed test of general relativity in a much stronger gravitational field than we've managed before.

If the magnetic field they detected extends to the surface of the black hole, it would be sufficient to explain the entire spectrum of emissions from the Milky Way's center."

Isn't the point of a black hole the escape velocity is > c? If so, how is it generating a magnetic field? Isn't the field composed of some kind of particles carrying the magnetic charge? (photons maybe?) If so, how is the magnetic force carrier escaping the black hole's gravity, or is the force carrier of magnetism immune to gravity?

And aren't electricity and magnetism pretty much mirror images of each other depending on relative motion?

If the magnetic field they detected extends to the surface of the black hole, it would be sufficient to explain the entire spectrum of emissions from the Milky Way's center."

Isn't the point of a black hole the escape velocity is > c? If so, how is it generating a magnetic field? Isn't the field composed of some kind of particles carrying the magnetic charge? (photons maybe?) If so, how is the magnetic force carrier escaping the black hole's gravity, or is the force carrier of magnetism immune to gravity?

And aren't electricity and magnetism pretty much mirror images of each other depending on relative motion?

The hole itself isn't generating a magnetic field, it's all the matter around the hole doing so as it`s compressed and spun.

Magnetism and gravity aren't at all similar (for one, magnetism is far stronger, but also drops more rapidly with distance). You may be thinking of acceleration, which is indeed indistinguishable from gravity.

As far as the shape of a black hole - is the artist's rendering correct? Looks like a squashed cylinder rather than a sphere.

Yep. The event horizon itself is a sphere* but the Milky Way has most of its mass all on one plane, and since all of the matter surrounding the hole comes from the MW, that means that it, too, will all be on one plane.

*I think. So far as I can recall the spinning of the hole doesn't change that.

Yep. The event horizon itself is a sphere* but the Milky Way has most of its mass all on one plane, and since all of the matter surrounding the hole comes from the MW, that means that it, too, will all be on one plane.

*I think. So far as I can recall the spinning of the hole doesn't change that.

I've been reading a book on this lately, but I'm by no means an astrophysicist. If I recall what I read correctly, the shape of the black hole's event horizon won't be affected by the fact that the matter is coming in mostly on the plane of the Milky Way*, but the spinning will affect the shape. The spinning can cause the event horizon to bulge around its equator (I would say "like our planet", but for all I know the mechanism might have more to do with warped space-time than centrifugal forces).

* Edit: To be clear, I'm not contradicting GDwarf -- event horizon vs. accretion disk, where the accretion disk is responsible for most of the "visible" effects of a black hole.

Gods I love the way in which we can use bits of information from one part of our patchy model to create experiments to test another piece. It's like a gigantic jigsaw puzzle. I imagine that in the 1500-1900 range, we were finding the borders of the puzzle, in the 1900-1985 range, we were filling in some of the more obvious sections of the picture ("That's a piece of the sky. It must go up here"), and now we're filling in some of the more difficult bits. I wonder if we'll hit a point at which there are so few pieces left (unknowns in our model of how the universe works) that we'll see a flood of discoveries as we use our nearly complete model to fill in the last remaining gaps.

Gods I love the way in which we can use bits of information from one part of our patchy model to create experiments to test another piece. It's like a gigantic jigsaw puzzle. I imagine that in the 1500-1900 range, we were finding the borders of the puzzle, in the 1900-1985 range, we were filling in some of the more obvious sections of the picture ("That's a piece of the sky. It must go up here"), and now we're filling in some of the more difficult bits. I wonder if we'll hit a point at which there are so few pieces left (unknowns in our model of how the universe works) that we'll see a flood of discoveries as we use our nearly complete model to fill in the last remaining gaps.

The analogy probably falls apart at the end, but I'm still hopeful.

I think we're so far from "the end" that most of our progress in the form of "Oh, these pieces don't actually fit together. If we tear this bit apart, and reassemble the pieces like so, then we have General Relativity, and now all these other pieces will fit properly..." or, "Hey, I started assembling this little cluster of pieces in a new part of the puzzle. Got any pieces that fit with this?"

I think the "end" might be better suited to a crossword puzzle analogy, where we're left staring at a mostly completed crossword, but we can't get the final words without undoing some existing solutions that were almost right, but not exactly right. The end will probably consist of fixing lots of little snarl-ups in smaller and smaller parts of the puzzle, where fixing up each section gains us a little more completion, but with less and less impact on the overall puzzle.

Working within the limits of General Relativity, can there be a force > c? Wouldn't that inherently predict velocity > c?

Example: If a super massive black hole has a gravitational force > c, therefore an object would have the potential to be "pulled" (or accelerated) to a velocity > c.

c is a speed, not a force. That sort of makes your question a bit nonsensical. Please don't think that I'm just being picky, it's just hard to answer a question that on some level doesn't make sense.

No offense taken. But velocity of an object precludes a force that accelerated the object to that velocity. Perhaps I should have stated the question differently. If there is a gravitational force that is greater than the force that accelerates an object to c, could it therefore be concluded that an object could reach a velocity greater than c?

Keep in mind, I'm not questioning the validity of General Relativity, only the validity that any source, including a supermassive black hole, has a force (gravitational or otherwise) so great as to reverse photons traveling at c.

Gods I love the way in which we can use bits of information from one part of our patchy model to create experiments to test another piece. It's like a gigantic jigsaw puzzle. I imagine that in the 1500-1900 range, we were finding the borders of the puzzle, in the 1900-1985 range, we were filling in some of the more obvious sections of the picture ("That's a piece of the sky. It must go up here"), and now we're filling in some of the more difficult bits. I wonder if we'll hit a point at which there are so few pieces left (unknowns in our model of how the universe works) that we'll see a flood of discoveries as we use our nearly complete model to fill in the last remaining gaps.

Working within the limits of General Relativity, can there be a force > c? Wouldn't that inherently predict velocity > c?

Example: If a super massive black hole has a gravitational force > c, therefore an object would have the potential to be "pulled" (or accelerated) to a velocity > c.

c is a speed, not a force. That sort of makes your question a bit nonsensical. Please don't think that I'm just being picky, it's just hard to answer a question that on some level doesn't make sense.

No offense taken. But velocity of an object precludes a force that accelerated the object to that velocity. Perhaps I should have stated the question differently. If there is a gravitational force that is greater than the force that accelerates an object to c, could it therefore be concluded that an object could reach a velocity greater than c?

Keep in mind, I'm not questioning the validity of General Relativity, only the validity that any source, including a supermassive black hole, has a force (gravitational or otherwise) so great as to reverse photons traveling at c.

There is no such thing as a force that can accelerate a mass to c. Photons have no rest mass and therefore travel at c to begin with; nothing with a rest mass can travel at c because it takes an infinite amount of energy to get there. (E=Mc^2 is actually incomplete; this video explains.)

No offense taken. But velocity of an object precludes a force that accelerated the object to that velocity. Perhaps I should have stated the question differently. If there is a gravitational force that is greater than the force that accelerates an object to c, could it therefore be concluded that an object could reach a velocity greater than c?

Keep in mind, I'm not questioning the validity of General Relativity, only the validity that any source, including a supermassive black hole, has a force (gravitational or otherwise) so great as to reverse photons traveling at c.

"If there is a gravitational force that is greater than the force that accelerates an object to c, could it therefore be concluded that an object could reach a velocity greater than c"

As I understand it, there isn't any force that can accelerate an object to c, nor is there a gravitational force exceeding that (non-existent) force. Objects with mass can never reach c. Things like light, which propagate at c, aren't accelerated to that speed. It's just the speed at which it travels, from the moment it's created.

"... has a force (gravitational or otherwise) so great as to reverse photons traveling at c."

This is something I didn't understand until recently, and perhaps I still don't *really* understand. But here goes. Gravity doesn't directly affect light. A photon of light travelling away from a black hole doesn't get slowed down, then pulled back in. Instead, what's happening is that the incredible gravity of a black hole warps space-time so much so that a photon of light travelling away from a black hole will travel in a straight line, at the speed of light, but that "straight line" itself is bent to the point where the photon ends up back in the black hole.

A really great book on this topic is Black Holes and Time Warps. It's very approachable, with nary an equation to be found, and is very well written. It's not just about the topic, it's also about the history of the science.

Gods I love the way in which we can use bits of information from one part of our patchy model to create experiments to test another piece. It's like a gigantic jigsaw puzzle. I imagine that in the 1500-1900 range, we were finding the borders of the puzzle, in the 1900-1985 range, we were filling in some of the more obvious sections of the picture ("That's a piece of the sky. It must go up here"), and now we're filling in some of the more difficult bits. I wonder if we'll hit a point at which there are so few pieces left (unknowns in our model of how the universe works) that we'll see a flood of discoveries as we use our nearly complete model to fill in the last remaining gaps.

The analogy probably falls apart at the end, but I'm still hopeful.

I think we're so far from "the end" that most of our progress in the form of "Oh, these pieces don't actually fit together. If we tear this bit apart, and reassemble the pieces like so, then we have General Relativity, and now all these other pieces will fit properly..." or, "Hey, I started assembling this little cluster of pieces in a new part of the puzzle. Got any pieces that fit with this?"

I think the "end" might be better suited to a crossword puzzle analogy, where we're left staring at a mostly completed crossword, but we can't get the final words without undoing some existing solutions that were almost right, but not exactly right. The end will probably consist of fixing lots of little snarl-ups in smaller and smaller parts of the puzzle, where fixing up each section gains us a little more completion, but with less and less impact on the overall puzzle.

You're right, a crossword is probably a better analogy. I know it's unlikely we'll ever really understand all the mechanics of the universe, but I get so thrilled by articles like this that I kind of got carried away.

Gods I love the way in which we can use bits of information from one part of our patchy model to create experiments to test another piece. It's like a gigantic jigsaw puzzle. I imagine that in the 1500-1900 range, we were finding the borders of the puzzle, in the 1900-1985 range, we were filling in some of the more obvious sections of the picture ("That's a piece of the sky. It must go up here"), and now we're filling in some of the more difficult bits. I wonder if we'll hit a point at which there are so few pieces left (unknowns in our model of how the universe works) that we'll see a flood of discoveries as we use our nearly complete model to fill in the last remaining gaps.

The analogy probably falls apart at the end, but I'm still hopeful.

Actually its more like a jigsaw puzzle where every time you fit a piece you find 10 more new pieces.

Perhaps I've forgotten this over time, or perhaps it's because I only had 2 hours of sleep last night due to suicide shifts at work, but a question I've got is: How do you determine how much the polarized light from the magnetar has been rotated? Seems like you would have to know the initial polarization before it went through the magnetic field of the central black hole.

Magnetism and gravity aren't at all similar (for one, magnetism is far stronger, but also drops more rapidly with distance).

Drops more rapidly? It's been a while since high school physics, but are not both gravitation and Coulomb forces equally affected by the inverse square law? Also, the Coulomb constant is much larger than G.

Magnetism and gravity aren't at all similar (for one, magnetism is far stronger, but also drops more rapidly with distance).

Drops more rapidly? It's been a while since high school physics, but are not both gravitation and Coulomb forces equally affected by the inverse square law? Also, the Coulomb constant is much larger than G.

They're both affected.

Electrostatic charge has positive and negative components and is far stronger than gravity. This means large scale collections of charge are unlikely - if you managed to somehow spawn an Earth sized ball of protons that didn't decay or blow itself apart, it would very quickly attract electrons, negating its charge and effect. Mass doesn't have a sign, so isn't affected by negation. That's why ES dominates at small distances and gravity large - at small distances negation isn't an issue.

If you're annoyed by the name, I prefer to call it SGR J1745-29, but those damn radio astronomers have to call everything PSR, regardless.

The problem is more the numbers; 1745 -29 (right ascension 17 hours 45 minutes, declination 29 degrees south) describes a large blob of sky containing the centre of the galaxy, there are huge numbers of objects there and if they discover another pulsar it's going to be badly ambiguous.

It's about as useful a designator as 'Turkish-speaking person living near 40.7N 74.0W'; yes, Turkish isn't a majority language in the New York City metro area, but there are going to be an awful lot of people meeting that description!

If you're annoyed by the name, I prefer to call it SGR J1745-29, but those damn radio astronomers have to call everything PSR, regardless.

The problem is more the numbers; 1745 -29 (right ascension 17 hours 45 minutes, declination 29 degrees south) describes a large blob of sky containing the centre of the galaxy, there are huge numbers of objects there and if they discover another pulsar it's going to be badly ambiguous.

Yeah, I've heard this argument before (actually to be precise, I've won this argument before), but the number of SGRs in that region are small enough, and if someone discovers another SGR in that same RA/dec region, they'll call it something different obviously. You're not really supposed to use the name to figure out what it's coordinates are, that's what Simbad is for. Calling it PSR in my mind is worse because it just lumps it in with the many other pulsars, rather than calling it out for it's special status as a magnetar that displays SGR behavior.

Also historically sources have been called SGR if they were discovered by a soft gamma ray burst. This is how this source was discovered (by Swift's Burst Alert Telescope, although technically it was first discovered by Swift's X-ray Telescope, but it wasn't obvious it was an SGR from those data), so I have history of my side.

Anyway, the argument is moot, the radio astronomers will keep calling it PSR J1745-2900, and the X-ray astronomers will call it SGR J1745-29, e.g.:

If the magnetic field they detected extends to the surface of the black hole, it would be sufficient to explain the entire spectrum of emissions from the Milky Way's center."

Isn't the point of a black hole the escape velocity is > c? If so, how is it generating a magnetic field? Isn't the field composed of some kind of particles carrying the magnetic charge? (photons maybe?) If so, how is the magnetic force carrier escaping the black hole's gravity, or is the force carrier of magnetism immune to gravity?

And aren't electricity and magnetism pretty much mirror images of each other depending on relative motion?

I was actually wondering the exact same thing. Obviously the fundamental cause and substance of a black hole is exactly what would determine whether gravity would/could effect a magnetic field. But since our planet's magnetic field isn't warped toward our Sun I would have to assume that gravity has little to no effect on the magnetic field.

That said our magnetic field does have a tail due to the solar wind from the Sun. Thus high energy particles can effect the field so why can't gravity? It is a bit of a conundrum if you ask me.

Though we should keep in mind that gravity is considered a weak force (the weakest of the 4 forces). So bearing that in mind it should be relatively easy to find something that can counteract gravity I would think. Now I know there has been research into what creates and consists of both gravity and magnetic fields. I don't know if those questions have been resolved (I think they are still at least partially open). But once we understand both mechanisms fully we should then be able to find ways to utilize and manipulate both, ultimately bending them to our will.

Anyway, long story short I am curious to know if gravity has any affect on magnetism and if so, what that would be.

Yeah, I've heard this argument before (actually to be precise, I've won this argument before), but the number of SGRs in that region are small enough, and if someone discovers another SGR in that same RA/dec region, they'll call it something different obviously.

I'm terribly sorry, I read SGR as the three-letter abbreviation for Sagittarius rather than as soft gamma repeater.